usb: gadget: rndis: don't use dev_get_stats

Since 2.6.36, spin_lock_bh has been added to dev_txq_stats_fold. So
dev_get_stats cannot be called from atomic context.

Replaced it by netdev_stats_to_stats64 to copy the stats directly. This
will work if the device driver does not need txq tx stats folding.

http://lkml.org/lkml/2010/9/20/250



Change-Id: I715b00892beda56cc369139d6e2bdc9efb6bfe79
Signed-off-by: Benoit Goby <benoit@android.com>